Navigational impairments force Alzheimer's disease (AD) patients to abandon driving and independent living. We hypothesize that navigational impairments in aging and AD reflect failures of the neural mechanisms supporting the spatial and temporal integration of visual cues. We suggest that specific navigational deficits reflect the reliance of particular navigational skills on different segments of the visual spatiotemporal spectrum. To test these ideas, we will characterize visual processing in aging and AD linked to three fundamental navigational behaviors: heading estimation, route perception, and path integration. We have previously linked optic flow perceptual deficits to navigational impairments in AD. These patients show combined impairments in the spatial and temporal integration of visual stimuli. We now propose a comprehensive test of optic flow's role in navigation by characterizing links between specific navigational skills and the spatiotemporal composition of optic flow stimuli. We will apply spatial and temporal filters to optic flow simulating self-movement as subjects engage in a series of two-alternative forced-choice tasks designed to measure their sensitivity to the direction, speed, and distance of simulated self-movement. These studies will be coupled with detailed navigational and neuropsychological tests. In three Specific Aims, we will study: 1) Heading estimation, as the ability to indicate the direction, speed, and distance of self-movement. 2) Route perception, as the ability to discriminate relative directions, speeds, and distances traveled on a two-segment route. 3) Path integration, as the ability to combine directions, speeds, and distances for an optimal route of return to the origin. In each aim, we will manipulate the spatiotemporal composition of the stimuli to probe the relationship between perception and navigation. This work will lead to a new understanding of the perceptual foundations of navigation. We will test the relationship between spatiotemporal integration and perceptual processing for different navigational tasks in the context of navigational impairments in aging and AD. This will contribute to approaches for monitoring the progression of AD, evaluating patients'safety for functional independence, and assessing new treatments.